This section is intended to introduce the reader to various aspects of art that may be related to various aspects of the presently described embodiments. This discussion is believed to be helpful in providing the reader with background information to facilitate a better understanding of the various aspects of the described embodiments. Accordingly, it should be understood that these statements are to be read in this light and not as admissions of prior art.
In most offshore drilling operations, a wellhead at the sea floor is positioned at the upper end of the subterranean wellbore lined with casing, a blowout preventer (“BOP”) stack is mounted to the wellhead and a lower marine riser package (“LMRP”) is mounted to the BOP stack. The upper end of the LMRP typically includes a flex joint coupled to the lower end of a drilling riser that extends upward to a drilling vessel at the sea surface. A drill string is hung from the drilling vessel through the drilling riser, the LMRP, the BOP stack and the wellhead into the wellbore.
During drilling operations, drilling fluid, or mud, is pumped from the sea surface down the drill string, and returns up the annulus around the drill string. In the event of a rapid invasion of formation fluid into the annulus, commonly known as a “kick,” the BOP stack and/or LMRP may actuate to help seal the annulus and control the fluid pressure in the wellbore. In particular, the BOP stack and the LMRP include closure members, or cavities, designed to help seal the wellbore and prevent the release of high-pressure formation fluids from the wellbore. Thus, the BOP stack and LMRP function as pressure control devices.
For most subsea drilling operations, hydraulic fluid for operating the BOP stack and the LMRP is provided using a common control system physically located on the surface drilling vessel. However, as a backup, or even possibly a primary means of operation, hydraulic fluid accumulators located subsea are filled with hydraulic fluid under pressure. The amount and size of the accumulators depends on the anticipated operation specifications for the well equipment.
An example of an accumulator includes a piston accumulator, which includes a hydraulic fluid section and a gas section separated by a piston movable within the accumulator. The hydraulic fluid is placed into the fluid section of the accumulator and pressurized by injecting gas (typically inert gas, e.g., nitrogen) into the gas section. The fluid section is connected to a hydraulic circuit so that the hydraulic fluid may be used to operate the well equipment. As the fluid is discharged, the piston moves within the accumulator under pressure from the gas to maintain pressure on the remaining hydraulic fluid until full discharge.
The ability or capacity of the accumulator to operate a piece of equipment depends on the amount of hydraulic fluid in the accumulator and the pressure of the gas. Subsea accumulators may be charged with pressure by a pumping system on the surface drilling vessel and are limited by the pressure capacity rating for the system, e.g., 5,000 psi (34,473.79 kPa). However, as water depth increases, accumulators become less efficient. For example, if the drilling site lies deeper than 10,000 ft (3,048 m) of water, the hydrostatic pressure will exceed 5000 psi. At that depth, colder temperatures reduce the nitrogen precharge in the accumulators. Therefore, the hydraulic fluid must be pumped into the accumulator at the correct working pressure and with the hydrostatic pressure taken into consideration. Consequently, with high operating pressures and a reduced precharge from colder temperatures, only a small percentage of the nominal volume is available for usable pressurized fluid.
The ability to charge up the subsea accumulators to pressure greater than 5,000 psi (34,473.79 kPa) can improve the accumulator volumetric efficiency. Systems may be built to include a special hotline reel with the ability to provide supply fluid under higher pressure than 5,000 psi (34,473.79 kPa) and this reel is connected to the BOP stack. This hotline system requires a separate hose reel located on the surface in the rig moon pool area, which can already be crowded with other equipment. There must also be a separate set of high pressure pumps to generate the high pressure fluid. The hose is run down the riser and a hydraulically operated stab assembly is used to allow communication of the high pressure fluid from the BOP stack LMRP to the lower stack where the subsea accumulators are located. This additional equipment requires maintenance and inspection to continue to operate reliably.